Method for manufacturing a corrugated pipe, and a corrugated pipe manufactured by the method

ABSTRACT

The invention relates to a method for manufacturing a multi-layer corrugated pipe and to a corrugated pipe manufactured by the method and comprising an inner tube (2), a corrugated outer tube (5) and a plastic layer therebetween. Separate layers (3,6) of plastic are formed on the outer surface of the inner tube (2) and the inner surface of the outer tube (5), said separate layers of plastic being affixed to one another at least at the web portions (5a) of the corrugations of the outer tube (5) when the inner tube and outer tube are connected.

The present invention relates to a method for manufacturing a corrugatedpipe and to a corrugated pipe manufactured by the method, being formedof an inner tube, a corrugated outer tube and a plastic layertherebetween.

Finnish Patents 60 825, 74 654 and 77 405, for example, teach productionlines for corrugated pipe, in which plastified plastic mass is extrudedwith a nozzle between movable forming means pressed against one anotherto produce a continuous corrugated pipe. This solution provideshigh-quality, commercially advantageous corrugated pipe products.

To save in weight and expensive raw materials, corrugated pipe is alsomanufactured as double-walled pipe, so that the inner pipe consists of aconventional smooth-surfaced tube and the outer pipe of a corrugatedtube formed thereon, cf. e.g. European Patent 385 465. The use ofrecycled plastic has been investigated in pipes manufactured inaccordance with this technology so that the outer pipe has been made ofrecycled plastic. Thus corrugated recycled plastic can as such replacethe outer tube of European Paten 385 465, or it can be employed in afoamed state to form a corrugated profile filled with foam on the innertube.

The long-term mechanical strength of recycled plastic, which may be poorand sometimes unpredictable, constitutes the drawback of these pipes.For this reason, such tubes cannot be used in applications that requirethat the pipe be manufactured to design specifications of standardtubes.

Solutions are also known in which a layer of recycled plastic isdisposed between a smooth inner tube and a corrugated outer tube, cf.German Offenlegungsschrift 41 28 654. This solution is attended by thedrawback that in pressing the outer tube against the layer of recycledplastic to produce a bond between the layers, substantial flashes ofdisplaced recycled plastic are formed at the web portions of thecorrugations in the outer tube. These flashes have no part in theload-carrying capacity of the pipe and thus impart an unnecessaryadditional weight to the pipe.

Lastly, the pipe disclosed in U.S. Pat. No. 5,324,557 provides anexample of `ribbed pipe` incorporating recycled plastic. In this pipe, alayer which may be of recycled plastic is provided between the outertube comprising ribs and the inner tube. As is stated in this reference,corrugated tube differs from ribbed tube by structure andcharacteristics. Since the ribs in a ribbed tube are always filled, theabove flash problem encountered with recycled plastic will not arise.

It is an object of the present invention to provide a method formanufacturing a corrugated pipe and a corrugated pipe manufactured bythe method which avoid the above-stated drawbacks and which enable a wayof employing recycled plastic in corrugated pipes which is moreefficient than has previously been thought possible, and also flexible.The method of the invention is characterized in that separate layers ofplastic are formed on the outer surface of the inner tube and the innersurface of the outer tube, said separate layers of plastic being affixedto one another at least at the web portions of the corrugations of theouter tube when the inner tube and outer tube are connected.

The most significant advantage of the invention is that the outer tubecan be made thinner than previously, since a separate supporting layerincreasing its resistance against buckling in particular is provided onthe inner surface thereof. Furthermore, no unwanted flashes are producedwhen the inner and outer tube are brought together, but all plasticmaterial in the pipe takes part in carrying outer or inner loads.Further advantages are afforded by the fact that by using separatelayers, the layer thickness, characteristics and material can betailored individually. This affords a considerable advantage in that byadjusting the amount and density of foam, the ring stiffness of the pipecan be easily modified without changing the joint dimensions of thepipe, such as the outer and inner diameter. Design of conventionalcorrugated double-walled pipes is determined by the required ringstiffness and the fact that the corrugations shall not buckle when thepipe is loaded. When a low weight is striven for, the optimum pipegeometry can only be selected within a rather narrow range. Theadvantage of the invention resides in that the foam support layersafford a broader range within which the geometry can be varied while theouter dimensions of the pipe remain the same. In a preferred embodiment,when the foam is light and thin, the pipe has a stiffness of e.g. 4kN/m². By increasing the amount and density of foam, the stiffness canin the best case be quadrupled for demanding applications, while thegeometry remains the same. Thus the same joint dimensions can beemployed, i.e. the jointing needs of widely different pipes can be metwith the same seal and pipe joint. This is normally not possible, sinceeither the inner or outer diameter of the pipe changes materially indifferent stiffness classes, and it is desired to keep the weight low.Thus a less severe "punishment" for using the same geometry in differentstiffness classes is achieved with the use of supporting foam.

Other preferred embodiments of the method and the pipe of the inventionare characterized in that which is set forth in the appended claims.

FIG. 1 is a general view of a step of manufacturing corrugated pipes;

FIG. 2 shows an embodiment of a corrugated pipe of the invention;

FIG. 3 shows another embodiment of the corrugated pipe of the invention;and

FIG. 4 shows still another embodiment of the corrugated pipe of theinvention.

The corrugator shown in FIG. 1 generally comprises two superimposedpaths of rotation of mould parts 1, i.e. chill moulds moving in thedirection of the arrow (herein only the topmost path has been shown).The chill moulds impart to the corrugated pipe its outer shape. Thebasis functions of such production apparatus are obvious to thoseskilled in the art and will not be explained in detail in this context.In accordance with the invention, a smooth inner tube 2 and a layer 3,preferably of foamed recycled plastic, formed thereon are introducedfrom a coextruder, for instance, to a mandrel 4 of the corrugator.Simultaneously an outer tube 5 and a layer 6, likewise preferably offoamed recycled plastic, formed on the inner surface of the outer tube 5are brought against the mould surfaces of the moving chill moulds 1.Layers 5 and 6 can be retained against the mould surfaces by applying apositive pressure in the space between the mandrel 4 and the layersand/or generating a negative pressure between the layers and the chillmoulds 1, as disclosed in U.S. Pat. No. 4,865,797.

The plastic layers 3 and 6 formed on the outer surface of the inner tube2 and on the inner surface of the outer tube 5 are affixed to oneanother in the corrugator at least at the web portions 5a of the outertube 5 of the layer. Since layers 3 and 6 are in a soft state andpossible being foamed when they are brought together, they are affixedto one another without any need for pressing, thus avoiding formation ofunwanted flashes.

FIG. 2 shows an embodiment of the pipe of the invention on a largerscale, wherein the foamed plastic layer 3 formed on the outer surface ofthe inner tube 2 has a different porosity structure than the foamedplastic layer 6 formed on the inner surface of the outer tube 5. Byusing comparatively small-pore foaming in forming layer 6, a stifferstructure is achieved in the pipe corrugations than when using a foamingtechnique producing a large amount of foam with large pores. This is ofadvantage, since on account of the stiffer structure the thickness ofthe more expensive outer tube and the risk of buckling of thecorrugations in the structure can be reduced. Also, the stiffer andthicker the wall is the less the tendency of the pipe wall to buckle.Plastic layer 3 is not subject to the danger of buckling, and thus thelatter foaming technique may be employed for that layer.

One problem with the manufacturing technology is that the mechanicalstrength of the foam decreases more rapidly than its density in relationto the degree of foaming. In other words, if the density of the foam isfor example half of the density of the plastic, its mechanicalproperties have perhaps only a third of their initial value. Inparticular, the long-term strength properties, such as creep resistance,are impaired. It has now unexpectedly been found that by cross-linking apolyolefin foam, such as polyethylene or polypropylene foam, the creepresistance properties can be substantially improved. Thus the propertiesof foam based on plastics waste, which has varying characteristics, canbe brought to a level that can be relied upon in design in view oflong-term strength properties.

The layers can be foamed by the conventional chemical method in which afoaming chemical is added to the plastic, with the result that theplastic is foamed when it squirts out into the foaming zone which has alower pressure.

An alternative method is to mix gas mechanically into the layer to befoamed in a nozzle or extruder. When the pressure in the forming zone inthe corrugator is relieved, the gas expands and foams the plastic.

Cross-linking agents, such as peroxides or azo compounds, may if desiredbe added into plastic forming foamed plastic layers to produce chemicalcross-linking. The characteristics of a cross-linked foamed plasticlayer are homogenized, and thus the quality and creep properties areimproved. The degree of cross-linking can be easily adjusted inaccordance with the required characteristics. A conventionalcross-linked polyethylene pressure pipe requires a degree ofcross-linking of about 70% in order that good strength properties may beachieved. In sewer pipes, a degree of cross-linking of 30-60% alreadyimproves the creep properties considerably.

FIG. 3 shows on a large scale another embodiment of the pipe of theinvention, wherein a foamed plastic layer 7 is formed on the innersurface of the outer tube 5. The plastic layer fills up the spacebetween the ribs of the outer tube and the inner tube substantiallyentirely. Thus, when the plastic layer 7 is foamed, it is easy toproduce a positive pressure effectively pressing the ribs of the outertube against the mould surfaces of the chill moulds 1, simultaneouslyproviding a corrugated pipe the outer surface of which has a very goodfinish. The risk of buckling of the corrugations has practically beeneliminated in such a structure, which fact is of special importance whena comparatively stiff rubber seal is placed between the ribs. In aconventional double-walled pipe, the corrugations will easily yield attheir lateral sides, with the result that the surface pressure of therubber ring seal decreases.

In the embodiment of FIG. 4, the foam layer 7 is hollow at thecorrugation.

It is obvious to those skilled in the art that the invention is notrestricted to the above examples, but the different embodiments of theinvention may vary within the scope of the appended claims.

We claim:
 1. In a method for manufacturing a mulit-layer corrugated pipeformed with an inner tube, a corrugated outer tube and a first layer ofplastic therebetween, the improvement wherein the pipe is also formedwith a second layer of plastic, the first and second layers of plasticbeing formed on an outer surface of the inner tube and an inner surfaceof the outer tube respectively or vice versa, the improvement furthercomprising affixing the first and second layers to one another at leastat web portions of corrugations of the outer tube with the inner tubeand outer tube connected.
 2. The method as claimed in claim 1, whereinthe plastic in each of the first and second layers is recycled plastic.3. The method as claimed in claim 1, wherein the plastic in each of thefirst and second layers is foamed plastic.
 4. The method as claimed inclaim 3, wherein a compound is added to the first or second foamedplastic layer or both to produce chemical cross-linking of said layer orlayers.
 5. The method as claimed in claim 3, wherein the plastic of thelayer formed on the inner surface of the outer tube is foamed to fill upsubstantially an entire space between the inner surface of the outertube and the layer formed on the outer surface of the inner tube.
 6. Themethod as claimed in claim 4, wherein the plastic of the layer formed onthe inner surface of the outer tube is foamed to fill up substantiallyan entire space between the inner surface of the outer tube and thelayer formed on the outer surface of the inner tube.
 7. The method asclaimed in claim 5, wherein the plastic of the first or second layerformed on the inner surface of the outer layer is foamed with thegeneration of a positive pressure that presses ribs of the outer tubeagainst a corrugation mold.
 8. The method as claimed in claim 3, whereinthe foamed plastic in the first layer is different from the foamedplastic in the second layer such that the first layer has a differentporosity structure than the second layer.
 9. The method as claimed inclaim 4, wherein the formed plastic in the first layer is different fromthe foamed plastic in the second layer such that the first layer has adifferent porosity structure than the second layer.
 10. The method asclaimed in claim 5, wherein the foamed plastic in the first layer isdifferent from the foamed plastic in the second layer such that he firstlayer has a different porosity structure than the second layer.
 11. In amulti-layer corrugated pipe comprising an inner tube, a corrugated outertube and a first layer of plastic therebetween, the improvement whereinthe pipe also comprises a second layer of plastic, the first and secondlayers of plastic being formed on an outer surface of the inner tube andan inner surface of the outer tube respectively or vice versa, saidfirst and second layers affixing the inner tube and outer tube to oneanother at least at web portions of corrugations of the outer tube. 12.The pipe as claimed in claim 11, wherein the plastic in each of thefirst and second layers is recycled plastic.
 13. The pipe as claimed inclaim 11, wherein the plastic in each of the first and second layers isfoamed plastic.
 14. The pipe as claimed in claim 13, wherein the firstor second foamed plastic layer or both is cross-linked.
 15. The pipe asclaimed in claim 13, wherein the plastic of the layer formed on theinner surface of the outer tube fills up substantially an entire spacebetween the inner surface of the outer tube and the layer formed on theouter surface of the inner tube.
 16. The pipe as claimed in claim 13,wherein the first layer of foamed plastic has a different porosity thanthe second layer of foamed plastic.
 17. The pipe as claimed in claim 14,wherein the first layer of foamed plastic has a different porosity thanthe second layer of foamed plastic.
 18. The pipe as claimed in claim 15,wherein the first layer of foamed plastic has a different porosity thanthe second layer of foamed plastic.
 19. The pipe as claimed in claim 11,wherein the pipe has a ring stiffness of 4 kN/m² when the first andsecond layers have approximately the same thickness as the inner andouter tubes.
 20. The pipe as claimed in claim 11, wherein the pipe has astiffness with the corrugation filled with high-density plastic that isat least twice the stiffness of an unfilled pipe.